1. Field of the Invention
The invention generally relates to a method and a circuit for processing pulses of a pulse train such that tails of pulses which precede a pulse to be processed and/or pile-up pulses which are piled up on the tails of preceding pulses to be processed are eliminated. Thereby the pulses can be processed in analog or digital form.
The invention is particularly suited for use in a radiation detector, such as a scintillation camera for detecting gamma rays.
2. Description of the Prior Art
Radiation detectors are widely used as diagnostic tools for analyzing the distribution of a radiation-emitting substance in an object under study, such as for the nuclear medical diagnosis of a human body organ. A typical radiation detector is a commercial version of the Anger-type scintillation camera, the basic principles of which are described in Anger U.S. Pat. No. 3,011,057.
Such a scintillation camera can take a "picture" of the distribution of radioactivity throughout an object under investigation, such as an organ of the human body which has taken up a diagnostic quantity of a radioactive isotope. As individual gamma rays are emitted from the distributed radioactivity in the object and pass through a collimator, they produce scintillation events in a thin planar scintillation crystal. The events are detected by photodetectors positioned behind the crystal. Electronic position computation circuitry translates the output pulses of the photodetectors into X and Y coordinate signals which indicate the position in the crystal of each event and a Z signal which indicates generally the energy of the event and is used to determine whether the event falls within a preselected energy window. A picture of the radio-activity distribution in the object may be obtained by coupling the X, Y and Z signals which fall within the preselected energy window to a display, such as a cathode ray oscilloscope which displays the individual scintillation events as spots positioned in accordance with the coordinate signals.
In commercial scintillation cameras, the position computation circuitry has normally been constructed so that processing of output pulses of the photodetectors emanating from different detected radioactive events have necessarily been performed sequentially. That is, pulses could be accepted and processed from only one quanta of radiation at a time. With regard to tails of ANTE or "A" pulses, i.e. tails of pulses which precede a pulse to be processed, prior art uses circuitry to check that the baseline has returned to zero within finite limits before a pulse is allowed to be processed. If the baseline exceeds that limit, an otherwise perfectly valid pulse is not accepted, but does impair the base line level for subsequent pulses. In the case of POSTE or "P" pulses, i.e., invalid pile-up pulses which are pile-up on the tails of preceding valid pulses to be processed, prior art such as disclosed in U.S. Pat. No. 3,984,689 uses circuitry which detects if a pile-up pulse occurs while a valid pulse is being processed and then dumps both, the valid pulse and the invalid pile-up pulse.
Other prior art such as disclosed in U.S. Pat. Nos. 4,024,398 and 4,051,373 include compensated delay line clipping circuitry for narrowing data representing pulses in order to minimize data loss due to pile-up and a restoring circuit for minimizing baseline fluctuation of a data signal. The circuit of U.S. Pat. No. 3,752,988 utilizes a combination of energy discrimination and delay logic for eliminating pulse-pile-up by delaying the processing of any pulses that would otherwise follow too soon after any given pulse.
All prior art methods and circuits are relatively complicated in technical design and they only provide pulse processing rates which are relatively slow.